Geoff Coope asked about the close: behaviour of the NSPopover when entering Mission Control in the comments of my previous post Epiphany-for-fixing-nspopover .
He had noticed that the Popovers did not close when you entered into Mission Control.
This was not something I had noticed as I had never gone into Mission Control with a popover open. But he was right and not only in Mission Control, entering Launchpad also does not close the Popover.
I had just been using a notification to control a NSDocuments behaviour and had an idea that the same thing could work.